The present invention generally relates to a system for sampling an analog video information signal having a PAL format and, more particularly, is directed to dropout and/or error concealment of the resulting digital data stream. Specifically, the sampling is accomplished at 45.degree., 135.degree., 225.degree. and 315.degree. with respect to the U color difference component of the chrominance.
The present invention is intended for use with a video recording and reproducing apparatus such as a magnetic tape recording and reproducing apparatus, although other types of recording and reproducing apparatus may advantageously incorporate the present invention as will be apparent from the following description. The present invention utilizes apparatus for sampling an analog video information signal with a PAL format at four times the frequency of the subcarrier component of the signal. The resulting digitized samples provide significant advantages in terms of the dynamic range of the sampling system and also increase the phase accuracy and therefore the resulting accuracy of the color information while providing a simple and effective method for error compensation of defective and/or missing data.
Recent research and development has been directed toward digital recorders which do not employ FM recording, but instead record digital words that represent samples which were taken of the analog video information signals. The digital words are recorded and reproduced and subsequently converted back to analog signals. There are significant advantages in recording digital signals as opposed to FM recording, among which reside in the fact that during playback or recording, the video signal is not degraded by head to tape spacing losses, Moire, noise and velocity errors. The digital signals permit error correction to be more accurately achieved using data from preceding and succeeding horizontal video lines for correction of defective or missing data words that result from unsuccessful recovery of the data, often referred to as dropouts, wherein an imperfection in the magnetic tape or other problem results in the loss of the signal for very short intervals during reproducing.
Another inherent advantage of digital video recording and reproducing apparatus is that the sampling of the analog signal can be done at a precise phase relationship and accomplished before recording. Because the phase relationship is set before recording, it will not be changed during playback by the presence of any velocity errors. Velocity errors and the like can not affect the phase relationship once sampling is done. It should be appreciated that the accuracy of the phase relationship is determinative of the resulting accuracy of the color information that is recorded and reproduced. In other words, if the sampling clock which determines the actual phase location of the samples taken during each subcarrier cycle is incorrect, then the resulting errors in the color information will be fixed in the samples taken thereby detrimentally affecting the resulting reproduced color information. While they will not change during reproducing as a result of velocity errors and the like, any error that is inserted during the sampling process will occur during reproducing.
Since the sampling clock controls the phase locations at which the samples are taken, it is extremely important that the phase of the sampling clock be made to accurately follow the analog video information signal. The sampling clock is controlled by the known phase relationship to a signal component which is present in the cycles of burst located on the back porch of a blanking interval subsequent to the horizontal sync pulse of each horizontal line. Since the sampling clock is used to sample the burst component of the chrominance subcarrier as well as the active video information for each video line, the samples of the burst component can be stored and thereafter examined to determine if any phase error exists between the sampling clock and the burst component. By appropriately choosing the sampling locations of the burst component, the accuracy of the phase synchronization can be increased, and the dynamic range of the sampling system can be effectively increased as well. Additionally, in accordance with the present invention, by judiciously choosing the sampling locations of the bust component, improved error concealment can be achieved. As will be more fully described, the improved error concealment is a direct result of the choice of the sampling phase locations.
Accordingly it is an object of the present invention to provide an improved sampling apparatus which samples a PAL format video information signal at a rate of four times the frequency of the chrominance subcarrier at locations which are oriented at 45.degree. relative to the U and V vector axes of the PAL signal which permits the use of an improved error concealment technique where a substitute digital data word sample can be obtained and substituted for a defective or missing sample without performing any arithmetic computation.